ABSTRACT: One hundred and three natural populations of Medicago L. were collected in Spain, mainly from roadsides, non-irrigated or grazed lands. This germplasm was evaluated at Montpellier (France) with control cultivars and Spanish landraces under completely random block design, replicated four times, and observed for 64 qualitative or quantitative characters. Differences between natural populations and cultivated controls are highly significant, but gene flow occurs between wild and cultivated compartment and hybrid populations were identified. On the basis of multivariable analysis, the accessions were grouped into four clusters depending on their proximity with the cultivated pool to facilitate their management and ex situ conservation. The relationship between environment of the site of collection and phenotypic characteristics of the natural populations was also discussed. Different policies of conservation of these genetic resources are suggested to avoid their disappearance even when they disappeared from the other regions of the western Mediterranean. Spanish wild pool of alfalfa, also called ‘Mielga’, appears of great interest for the breeding of alfalfa because it contains a large diversity of characteristics (prostrate habit, rhizomes) linked to tolerance to grazing or drought environment. With the need for more sustainable systems in agriculture, the erosion of natural habitats and the necessity for rehabilitation of degraded ecosystems, the importance of this wild pool is really inestimable.